CYP3A4 is the major P450 present in human liver and small bowel[unreadable] epithelial cells (enterocytes). The contribution of enterocyte CYP3A4[unreadable] to drug elimination is believed to be substantial. However,[unreadable] distinguishing the contributions of the intestinal vs. the liver enzyme,[unreadable] as well as the role of the transporter P-glycoprotein, has been[unreadable] problematic. We have developed experimental techniques that have[unreadable] enabled us to safely study liver and enterocyte CYP3A4 in living people.[unreadable] We have shown that there exists marked inter- and intraindividual[unreadable] variation in the activity of enterocyte CYP3A4. We have also shown that[unreadable] there is generally not a good intraindividual correlation between the[unreadable] relative activities of CYP3A4 in liver and intestine. To further[unreadable] investigate the mechanisms underlying these variations, we have chosen[unreadable] to intensively pursue our observation that some fruit juices (grapefruit[unreadable] juice and Seville orange juice) cause a marked and relatively rapid loss[unreadable] of enterocyte CYP3A4 by a mechanism that does not appear to involve loss[unreadable] of CYP3A4 mRNA. We have shown that 2 juice-derived furanocoumarins (FC[unreadable] s), 6',7'-dihydroxybergamottin (DHB) and a related dimer (FC726), cause[unreadable] selective loss of CYP3A4 in a novel human intestinal (Caco-2) cell[unreadable] monolayer culture system, and that DHB is a mechanism-based inactivator[unreadable] of CYP3A4. However, DHB and FC726 can not fully account for the effects[unreadable] of whole juice, and their effects may not be CYP3A4 selective. We will[unreadable] test the hypothesis that multiple FC s, which are quite ubiquitous in[unreadable] fruits and vegetables, cause mechanism-based inactivation and[unreadable] accelerated intracellular degradation of CYP3A4. To this end, we will[unreadable] characterize the time course of the effects of selected FC s on CYP3A4[unreadable] and on other major enterocyte P450s in human intestinal microsomes, in[unreadable] our Caco-2 cell system, and in healthy volunteers. We will also[unreadable] determine the effects of selected FC s on rates of synthesis and[unreadable] degradation of CYP3A4 in our cultured cells. Finally, we will test the[unreadable] hypothesis that some oral medication regimens that result in clinically[unreadable] important induction of CYP3A4 in liver do not induce the enzyme in[unreadable] enterocytes. The data obtained from the proposed studies should provide[unreadable] substantial insight into the basis for variations in CYP3A4 activity[unreadable] between people, within a person over time, and between liver and[unreadable] intestine. In addition, the identification of orally administered FC[unreadable] s that specifically ablate CYP3A4 activity in enterocytes (but not[unreadable] liver), or drugs that selectively induce CYP3A4 in liver (but not in[unreadable] enterocytes) would provide powerful research tools to assess the[unreadable] relative contributions of enterocyte and hepatic CYP3A4 to the[unreadable] disposition of xenobiotics in living people.